1. Field of the Invention
This invention relates generally to the processing of color video information, and more particularly, is directed to the processing of a digital color video signal, as in a digital video tape recorder (VTR).
2. Description of the Prior Art
In recording and reproducing a digital color video signal by means of a rotary head type VTR, random errors may result from head noise, tape noise or amplifier noise, or a burst error may be caused by a signal drop out. It is a recognized advantage of digital signal processing that erroneous data can be mathematically corrected by the inclusion in the recorded or transmitted data of redundant bits. For example, a well known scheme for correcting digital data involves dividing the latter into blocks and sub-blocks, each of which is recorded or transmitted along with a cyclic redundancy check (CRC) code and horizontal and vertical parity so that, upon reproduction or reception, an error in any such block can be detected from the respective CRC code and then corrected on the basis of the respective parity data. However, the addition to the recorded information data of the redundant bits representing the CRC code and the parity data necessarily increases the recording bit rate which is limited by the necessity of minimizing the consumption of tape. Therefore, the error correcting ability is limited by the acceptable redundancy, and may be exceeded when an error of substantial extent is encountered.
It has further been proposed to conceal an error in a transmitted or recorded video signal so that such error will not be noticed in the displayed picture by replacing the erroneous data with data of the immediately preceding line of the same field. Another existing error-concealing method involves replacement of the erroneous data with a mean value of data from the lines immediately preceding and following the line containing the error. Each of the foregoing error concealing methods obtains the signal which is to be substituted for the erroneous data from the data of the same field. However, since the television picture is formed by interlaced scanning, it will be appreciated that adjacent lines in the same field are spaced apart by a distance that is twice the distance between adjacent lines in the pictorial representation of the complete frame made up of two interlaced fields. Therefore, the data in immediately adjacent lines of such pictorial representation of two contiguous fields of the video signal have an even higher correlation therebetween.
In view of the above, it has been proposed by the assignee of this application to effect error concealment by replacing error-containing data in a line of one field with corresponding data in a line of the next previous field which, in the pictorial representation of the two fields, is positioned immediately adjacent the error-containing line so that the data used for concealing an error will bear a close resemblance to the original or correct data which it replaces. However, in the case of a color video signal, the phase of the chrominance component, that is, of the subcarriers of the color information, may not be the same at corresponding locations in immediately adjacent lines of the pictorial representation of two contiguous fields. In other words, even if the color information is the same at corresponding locations in the immediately adjacent lines, the polarities thereof may be relatively inverted.
For example, in the case of an NTSC color video signal, if error containing data in a line of one field is replaced with corresponding data in the same line of the next previous field, the line from which data is obtained for replacing the erroneous data may be disposed either above or below the error-containing line in a pictorial representation of the interlaced fields, depending upon whether such fields are from the same frame or from different frames. More particularly, if the error-containing data occurs in a line of the second field of a frame and is replaced with corresponding data in the same line of the next previous field, that is, the first field of the same frame, such line of the first field is positioned immediately below the error-containing line in the pictorial representation of the complete frame, and the color information of the data used to replace the error-containing data is the same as, and of the same polarity as the color information in the error-containing line. On the other hand, if the error-containing data occurs in a line of the first field of a frame so that the next previous field is the second field of the earlier frame, then the line of such next previous field from which the data is derived for replacing the error-containing data is positioned immediately above the error-containing line in the pictorial representation of the two fields and the color information in such line immediately above the error-containing line will be of inverted polarity in respect to the color information in the error-containing line.
Accordingly, in the case of an NTSC color video signal, if error-containing data in a line of one field is replaced by corresponding data from the same line in the next previous field, the color information of the data used for replacing the error-containing data may or may not have the same polarity as the latter. Furthermore, since the error-containing line of one field and the same line in the next previous field from which data is derived for replacing the error-containing data are spatially displaced from each other in the vertical direction on the pictorial representation of the two interlaced fields, a discontinuity occurs, particularly in respect to the luminance component, at the location of the replaced data. Consequently, if the data used for concealing or replacing the error-containing data in an NTSC color video signal are not further processed, both the luminance and chrominance components are adversely affected and an unnatural or distorted color picture results.
Similarly, in the case of a PAL color video signal, if error-containing data in a line of one field is replaced with corresponding data in the same line of the next previous field, which in a pictorial representation of the two fields, is positioned immediately adjacent the error-containing line, the color information of the line of such next previous field from which data is to be derived for replacing the error-containing data will either be of inverted polarity in respect to the corresponding color information in the error-containing line, or different color information will appear at the corresponding sampling points along the two next adjacent lines. The spatial displacement of the error-containing line and the same line of the next previous field from which data is drawn for replacing or concealing the error-containing data also results in discontinuities, particularly as to the luminance component, at the locations where errors have been concealed in a PAL color video signal. Accordingly, an error occurring in a line of one field of a PAL color video signal cannot be effectively concealed merely by replacing the error-containing data with corresponding data from the same line in the next previous field.
Furthermore, in a VTR, a rotary head repeatedly scans across a magnetic tape while the latter is longitudinally driven for recording a color video signal in successive parallel tracks extending obliquely across the tape. The rate at which the rotary head repeatedly scans the tape is controlled so that, for example, one field of video information is recorded in each track. In the normal reproducing mode, the tape speed is the same as that for recording and the rotary head is made to faithfully scan a single track during each field period so as to reproduce only the field of video information recorded therein. However, in a non-normal reproducing mode, for example, during high-speed reproducing, the speed at which the magnetic tape is driven is many times the normal tape speed so that, during a single field period, the tape scans across a number of tracks and, therefore, reproduces successive fragments of video information from respective different fields and/or frames of the recorded color video signal. If such reproduced video information is employed, as is, for producing a color television picture, the resulting changes in the polarity and type of color information provided at various times during a field period and the discontinuities, particularly in the luminance information caused by the spatial displacement of the lines of the successively scanned lines and/or fields, results in a distorted and unnatural picture.